Depletion mode MOSFETs for motor control applications

[Coolstep]

Hello everyone,
I am working on a bldc motor control application using STSPIN32G4-chip. To prevent the motor from spinning freely in the disabled state I want to open the low-side MOSFETs and turn the motor into generator. Due to a high gear rate, it is more than enough to prevent the motor from spinning. To achieve the same behavior at power fault I wanted to use depletion mode MOSFETs (normally open). Those are quite expensive but can be used for high power applications (such as IXYS IXTH16N20D2). High side MOSFETs would be normal NMOS parts.
Unfortunately, I could not find any mention of such an application on the internet (not even H-bridge for DC-motors). Is this idea inherently fraud? Do I miss something? Are there some other solutions other than using a relais?
Thank you for your help.

[ajb]

Shorting the windings doesn't really prevent the motor from moving, since it requires the motor to be moving to produce any resistance, so it really just limits the speed of rotation for a given force -- so it's a better solution for dynamic braking than static braking.  A mechanical brake is generally a better solution for static braking, and it has the extra benefit of keeping the motor from moving even if the drive circuitry is disconnected.  That, plus the fact that enhancement mode parts are far more common, with a wider variety of parts tailored to different power applications vs depletion mode at much lower prices (generally), is why you don't really see the solution you're proposing.

Using depletion mode parts in the bridge means you'll need a negative supply for the gate voltage, so the gate drive circuitry will be kind of unusual and would require a different sort of level shifting for the control signals.  It may be difficult or impossible to use standard bridge driver ICs for this, due to the inverted drive voltage. 
Single-channel gate drivers could probably be used with the right isolation/level shifting, so it's doable, but more complicated, and you lose the convenience and built-in protections of more integrated solutions. 

As an alternative, you could use a standard enhancement-mode bridge, and then use a pair of depletion-mode part across the motor terminals.  It's a few extra parts (including an isolated gate drive supply), but probably cheaper and easier overall.

[Benta]

I'm not sure I understand your question.
Do you really want to use the motor as a generator?
Or do you just want to brake it so it doesn't move?
In that case, just turning on the three lower MOSFETs in your bridge should do the job (high-side FETS off, of course). That would short all three windings on the motor.
Whether your IC supports that? No idea.

[thm_w]

there some other solutions other than using a relais?
Thank you for your help.

What is wrong with using a relay for no-power fault state? It is probably cheaper and simpler than your idea.

[Coolstep]

Wow, thank you very much for the replies.

I guess the negative voltage just killed the idea.

A mechanical brake is generally a better solution for static braking, and it has the extra benefit of keeping the motor from moving even if the drive circuitry is disconnected.

In our application I cannot add a brake, as we want to retrofit the existing solutions. We have an external emergency system that detects irregular behavior and disables everything. Putting the motor in a generator state in a power fault-situation gives us enough time to react. Static braking is not required.

"In that case, just turning on the three lower MOSFETs in your bridge should do the job (high-side FETS off, of course)." That IC supports that, but it does not work without power.

I had experiences with relay's reliability. Yesterday I tested a prototupe with 2 solid state switches: CPC1705Y. There is a depletion mode MOSFET inside.  I put them backto back and shorted 2 of 3 contacts of the motor. They worked like a charm. I am hinking about integrating them as small pcb-module to integrate in existing machines.

Your answers really helped me, thank you very much